Modifications for the ICOM - RogerK

Sub band PSK receive audio! You get Main FM. Grrr!Downlink 1200 bps PSK reception from the Sub band receiver is excellent, as too is Oscar-13's 400 bps telemetry signal.The only snag with these PacSats is doppler tracking the PSK signal. (See later).9600 bps DFM SatellitesUosat-22, KitSat-23 etc require the TXAudio to be applied direct to the transmit FM varactor. RXAudio must be picked off directlyfrom the FM discriminator, and the system is full duplex.Thus the internal MOD slider switch needs to be set to PACT. But that immediately picks up the wrong RX audio - from FM Main'sdiscriminator. For satellites we need FM Sub's discriminator output.Consequently you CANNOT operate the 9600 bps satellites with an Icom IC-820H "straight out of the box".There is a solution, but we're back to modifications I'm afraid.What you do is locate the Sub receiver discriminator IC20, pin 9 and fly that signal out on your own lead. This requires you toremove the big PCB called Main Unit, turn it over and do some fine re-work among the Sea of SMD. Alternatively you can pick upa downstream version of the signal without removing the PCB at the optional tone-squelch unit-B socket J20, third pin from the"J". Incidentally this signal is DC coupled to the discriminator chip, so you can implement closed-loop AFC externally using one ofthe many published circuits. The source impedance is 47K (R329).I checked Uosat-22 and KitSat-23 on Sub-band using this modification. The UO-22 "eye" is poor when it leaves the satellite, witha lot of LF flutter which has always made decoding difficult. But the 9600 bps performance of the Sub band receiver is so good itadds little extra aberration and data decoding was quite satisfactory. The KitSat-23 "eye" was wide open, and data detectionperfect.Since there is no AFC indication for Sub band, tracking the changing doppler shift unaided requires either very good judgement,an external system as above, or computer control.Oh, and once again set the mic gain to zero, or microphone sounds will be added to your transmission.9600 bps DFM TerrestrialTerrestrial 9600 bps packet works perfectly, "right outa the box".Over the last six years I've tested innumerable radios for 9600 bps operation. The Icom IC-820H now shares top place withKantronics' D4-10. (The latter is however 2-channel, crystal controlled and UHF only).9600 bps packet requires the TXAudio to be applied direct to the transmit FM varactor. RXAudio must be picked off directly fromthe FM discriminator, and the system is simplex.Thus the internal MOD slider switch needs to be set to PACT, which also selects RXAudio from FM Main's discriminator.The transmitter circuit's frequency response is from about 15 Hz to well beyond 6 kHz, so the outgoing signal has superb fidelity.If the drive signal exceeds 1.6 volts pk-pk, corresponding to about +/- 5 kHz deviation, modulation is switched off abruptly andstays off until you reduce the drive. A nice touch. The correct drive level is 1 volt pk-pk for +/- 3 kHz deviation and I confirmedthis by measurement.The FM Main and Sub receive circuits are similar. Main uses a pair of Icom part no. FL-211 crystal roofing filters (no spec) and amuRata SFH455E ceramic final filter; Sub uses a pair of FL-212 and the muRata CFW455E. The "E" suffix means 15 kHzbandwidth. The SFH types have particularly flat delay characteristics, and are pin compatible with the more common generalpurpose CFW series.Main's fidelity is outstanding, with a flat frequency and delay response to over 6 kHz. The "eye" was essentially perfect. You canbe mistuned by up to +/- 4 kHz before the "eye" starts to look mangled, and +/- 5 kHz if the packets are short. The Sub receiveris almost equally good, but you can't get at it without modifying the radio as described earlier.No quibble with Icom's claims here; 9600 bps simplex works 101%.PSK Satellite Doppler TrackingWhen using a 1200 bps PSK digital satellite such as FO-20, PacSat, Lusat or Weber it is essential that the PSK modem cancontrol the radio receive frequency in a closed loop fashion, preferably in small steps. The universal means of doing this is viathe Up/Down buttons of the microphone socket.The smallest step from the mic Up/Down buttons of the IC-820H is 100 Hz. This really is too big, as the sudden lurch from onefrequency to the next will invariably cause momentary loss of demodulator lock, with attendant corrupted characters.The Up/Down line is also accessible from the accessory socket, but it shares a pin with the ALC control. You select which from aninternal slide switch.